Clones of cells can be obtained from reconstructed cells which have components derived from human and mouse cell lines and strains. By combining viable nuclei and cytoplasms in the normal cell ratio and maintaining these cells "in vitro" we can follow the cell replication and growth potential of these experimental cells. The finite life-span of the normal diploid human fibroblast WI-38, and differentiated mouse and human cells are being studied by using the reconstruction techniques. The regulation of function and gene action can be evaluated, and modulation of cellular growth with regard to senescence and transformation is being attempted.